Ulnar Mononeuropathies: Potential Entrapment Sites

PROXIMAL ULNAR NERVE LESIONS

Chronic ulnar nerve elbow lesions primarily exist at the cubital tunnel or condylar groove. Patients with these lesions present with numbness and tingling, and/or sensory loss over the entire fifth and medial fourth fingers. The ulnar nerve does not provide sensory innervation until reaching the wrist; thus medial forearm paresthesias indicate a medial brachial plexus lesion or a C8 radiculopathy. Indolent ulnar damage presents with the classic “papal blessing hand,” with hyperextension of the fourth and fifth metacarpophalangeal (MCP) joints and flexion of proximal interphalangeal joints. This is due to unopposed flexion of the proximal and distal interphalangeal (PIP/DIP) joints because of the loss of function of the ulnar-innervated third and fourth lumbrical muscles, primarily extending the PIP/DIP joints. Unopposed contraction of radial nerve innervated extensor digitorum muscles keeps the proximal phalanxes extended. Although ulnar forearm muscle weakness is rarely apparent to patients, the medial flexor digitorum profundus for the fourth and fifth fingers’ distal PIP joints is weakened with nerve entrapment at the elbow.

Typically, ulnar nerve elbow entrapment occurs at the ulnar condyle, presumably after remote elbow trauma (tardy ulnar palsy), or just distal to the elbow joint (cubital tunnel syndrome). Although chronic ulnar neuropathies are relatively common, a precise etiology is often not identified. Constant pressure from leaning the elbow on a chair arm or desk top may predispose to such a condition. Uncommonly, idiopathic focal hypertrophic neuropathy, tumors, and hamartomas affect the ulnar nerve. Leprosy is responsible for the majority of chronic ulnar nerve lesions in economically underdeveloped countries. Elbow fractures or dislocations commonly produce acute ulnar nerve compromise. Other acute mechanisms include external compression in anesthetized patients, hemorrhage in hemophiliacs, intravenous fluid extravasation leading to a compartment syndrome, and burns.

DISTAL ULNAR NERVE LESIONS

Ulnar neuropathies at the wrist or palm are uncommon. Entrapment at the ulnar tunnel, that is, the Guyon canal, may occur after wrist fractures or with rheumatoid arthritis and ganglion cysts. Sensory symptoms may or may not develop. When a sensory loss is demonstrated on the dorsal medial hand, compatible with involvement of the dorsal ulnar cutaneous nerve, the lesion is proximal to the wrist. In contrast, when ulnarinnervated muscle weakness is identified as confined to the intrinsic hand muscles, that is, both the hypothenar abductors and opponens as well as the thumb adductors (without sensory loss), this localizes a pure motor distal ulnar neuropathy at or distal to the Guyon canal. The most distal pure motor ulnar lesion involves the deep ulnar motor branch within the palm. This is characterized by weakness of the adductor pollicis, the primary thenar muscle not innervated by the median nerve, as well as concomitant weakness of the first dorsal interosseous muscle. The preservation of fifth-finger abduction provides the key to localizing the compression site to the lateral palmar hypothenar eminence.

Primary palmar lesions usually result from repetitive hand injury, for instance, from bicycling or from occupations using tools requiring significant intermittent pressure over the distal ulnar motor fibers at the hypothenar eminence. Typically, these patients have difficulty adducting the thumb and index finger, leading to problems placing a key in a lock. When the inciting mechanism is discontinued, significant recovery frequently occurs.

DIFFERENTIAL DIAGNOSIS

Motor neuron disease (MND) is a primary consideration in patients presenting with asymmetric painless atrophy of the hand intrinsic muscles, with no associated sensory deficits. The presence of concomitant median-innervated thenar weakness and atrophy usually occurs in MND because these muscles similarly have C8, T1 innervation. Lower brachial plexus lesions characteristically include both motor and sensory dysfunction in multiple peripheral nerve and nerve root territories within the arm. Thoracic outlet syndrome is a rare medial plexus lesion mimicking an ulnar neuropathy. However, these patients have greater thenar than hypothenar muscle weakness. C8 radiculopathies are uncommon lesions; neck pain is very important in clinical differentiation from an ulnar neuropathy. However, weakness of non–ulnar-innervated C8 muscles (the thenar eminence, the flexor pollicis longus (FPL), and the extensor indicis proprius) with medial forearm numbness provides major diagnostic distinctions.

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